- BASICS OF DIGITAL CAMERA PIXELS -

The continuous advance of digital camera technology can be quite confusing
because new terms are constantly being introduced. This tutorial aims
to clear up some of this digital pixel confusion-- particularly for those who
are either considering or have just purchased their first digital camera.
Concepts such as sensor size, megapixels, dithering and print size are discussed.

THE PIXEL: A FUNDAMENTAL UNIT FOR ALL DIGITAL IMAGES

Every digital image consists of a fundamental small-scale descriptor: THE
PIXEL, invented by combining the words "PICture ELement."
Just as how pointillist artwork uses a series of paint blotches, millions of
pixels can also combine to create a detailed and seemingly continuous image.

Move mouse over each to select:

Pointillism (Paint Blotches)

Pixels

Each pixel contains a series of numbers which describe its color or intensity.
The precision to which a pixel can specify color is called its
bit or color depth. The more
pixels your image contains, the more detail it has the ability to describe.
Note how I wrote "has the ability to"; just because an image has more pixels
does not necessarily mean that these are fully utilized. This concept
is important and will be discussed more later.

PRINT SIZE: PIXELS PER INCH vs. DOTS PER INCH

Since a pixel is just a logical unit of information, it is useless for describing
real-world prints-- unless you also specify their size. The terms pixels
per inch (PPI) and dots per inch (DPI) were both introduced to relate
this theoretical pixel unit to real-world visual resolution. These terms
are often inaccurately interchanged (particularly with inkjet printers)-- misleading
the user about a device's maximum print resolution.

"Pixels per inch" is the more straightforward of the two terms. It
describes just that: how many pixels an image contains per inch of distance
in the horizontal and vertical directions. "Dots per inch" may seem deceptively
simple at first. The complication arises because a device may require
multiple dots in order to create a single pixel; therefore a given number of
dots per inch does not always lead to the same resolution. Using multiple
dots to create each pixel is a process called "dithering."

A device with a limited number of ink colors can play a trick on the eye
by arranging these into small patterns-- thereby creating the perception of
a different color if each "sub-pixel" is small enough. The example above
uses 128 pixel colors, however the dithered version creates a nearly identical
looking blend of colors (when viewed in its original size) using only 24 colors.
There is one critical difference: each color dot in the dithered image has to
be much smaller than the individual pixel. As a result, images almost
always require more DPI than PPI in order to achieve the same level of detail.
PPI is also far more universal because it does not require knowledge of the
device to understand how detailed the print will be.

The standard for prints done in a photo lab is about 300 PPI, however inkjet
printers require several times this number of DPI (depending on the number of
ink colors) for photographic quality. It also depends on the application;
magazine and newspaper prints can get away with much less than 300 PPI.
The more you try to enlarge a given image, the lower its PPI will become (assuming
the same number of pixels).

MEGAPIXELS AND MAXIMUM PRINT SIZE

A "megapixel" is simply a unit of a million pixels. If you require
a certain resolution of detail (PPI), then there is a maximum print size you
can achieve for a given number of megapixels. The following chart gives
the maximum 200 and 300 PPI print sizes for several common camera megapixels.

# of Megapixels

Maximum 3:2 Print Size

at 300 PPI:

at 200 PPI:

2

5.8" x 3.8"

8.7" x 5.8"

3

7.1" x 4.7"

10.6" x 7.1"

4

8.2" x 5.4"

12.2" x 8.2"

5

9.1" x 6.1"

13.7" x 9.1"

6

10.0" x 6.7"

15.0" x 10.0"

8

11.5" x 7.7"

17.3" x 11.5"

12

14.1" x 9.4"

21.2" x 14.1"

16

16.3" x 10.9"

24.5" x 16.3"

22

19.1" x 12.8"

28.7" x 19.1"

Note how a 2 megapixel camera cannot even make a standard 4x6 inch print
at 300 PPI, while it requires a whopping 16 megapixels to make a 16x10 inch
photo. This may be discouraging, but do not despair! Many will be
happy with the sharpness provided by 200 PPI, although an even lower PPI may
suffice if the viewing distance is large (see "Digital
Photo Enlargement"). Many wall posters assume that you will not be
inspecting them from 6 inches away, and so these are often less than 200 PPI.

CAMERA & IMAGE ASPECT RATIO

The print size calculations above assumed that the
camera's aspect ratio, or ratio of longest to shortest dimension,
is the standard 3:2 used for 35 mm cameras. In fact, most compact
cameras, monitors and TV screens have a 4:3 aspect ratio, while most
digital SLR cameras are 3:2. Many other types exist though: some
high end film equipment even use a 1:1 square image, and DVD movies
are an elongated 16:9 ratio.

This means that if your camera uses a
4:3 aspect ratio, but you need a 4 x 6 inch (3:2) print, then a lot
of your megapixels will be wasted (11%). This should be considered
if your camera has a different ratio than the desired print dimensions.

Pixels themselves can also have their own aspect ratio, although this is
less common. Certain video standards and earlier Nikon cameras have pixels
with skewed dimensions.

DIGITAL SENSOR SIZE: NOT ALL PIXELS ARE CREATED EQUAL

Even if two cameras have the same number of pixels, it does not necessarily
mean that the size of their pixels are also equal. The main distinguishing
factor between a more expensive digital SLR and a compact camera is that the
former has a much greater
digital sensor
area. This means that if both an SLR and a compact camera have the same
number of pixels, the size of each pixel in the SLR camera will be much larger.

Why does one care about
how big the pixels are? A larger pixel has more light-gathering
area, which means the light signal is stronger over a given interval
of time.

This usually results in an improved
signal to noise ratio (SNR), which creates a smoother and more detailed
image. Furthermore, the
dynamic range of the images (range of light to dark which the camera
can capture without becoming either black or
clipping highlights) also increases with larger pixels. This
is because each pixel well can contain more photons before it fills
up and becomes completely white.

Compact Camera Sensor

SLR Camera Sensor

The diagram below illustrates the relative size of several standard sensor
sizes on the market today. Most digital SLR's have either a 1.5X or 1.6X
crop factor (compared to 35 mm film), although some high-end models actually
have a digital sensor which has the same area as 35 mm. Sensor size labels
given in inches do not reflect the actual diagonal size, but instead reflect
the approximate diameter of the "imaging circle" (not fully utilized).
Nevertheless, this number is in the specifications of most compact cameras.

Why not just use the largest sensor possible? The main
disadvantage of having a larger sensor is that they are much more expensive,
so they are not always beneficial.

Does all this mean it is bad to squeeze more pixels into the same sensor
area? This will usually produce more noise, but only when viewed at
100% on your computer monitor. In an actual print, the higher megapixel
model's noise will be much more finely spaced-- even though it appears noisier
on screen (see "Image
Noise: Frequency and Magnitude"). This advantage usually offsets any
increase in noise when going to a larger megapixel model (with a few exceptions).